I've got to go with Gengar. I love the whole Gastly line, as well as most Ghost-type Pokemon in general. Though Gen III really dropped the ball on their ghosts... Banette and Dusclops? Meh. In Gen V I'm a fan of Chandelure and the Litwick line. But really, there are a lot of amazing Pokemon so it's hard to pick a favorite. I'll always have a soft spot for Venusaur, my first starter. Ironically he was also the first holgographic Pokemon card I ever got. My 11-year-old self was convinced it was destiny. :)